deformation

deformation - DEFORMATION 1 04/03/12 MOUNTAINS FAULTS Fig....

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Unformatted text preview: DEFORMATION 1 04/03/12 MOUNTAINS FAULTS Fig. 11.00 FOLDS DEFORMATIONTEPHEN MARSHAK S PLANET EARTH 3 Deformation Deformation Change in 1. Location 1. 2. Orientation 2. 3. Shape or volume 3. Due to applied stress 04/03/12 Orientation Fig. 11.05b STEPHEN MARSHAK DIP Structure orientation Structure orientation 5 FOR A PLANE Strike ­ angle of imaginary horizontal line on plane, measured from north Dip ­ maximum angle of plane’s slope FOR A LINE Bearing ­ compass angle of a line Plunge ­ angle between line and horizontal 04/03/12 Tilted sedimentary layers Strike Strike Strike Dip Dip Some key terms 7 Stress ­ Force per unit area acting on a rock to change shape and/or volume Strain ­ Change in shape or volume from stress Main kinds of stress Compression ­ shortening Tension ­ stretching Shear ­ moves sideways 04/03/12 d Strain - change in shape or volume from stress Take a Fossilized Shell… This is a great example of a strain indicator! 9 04/03/12 Deformation Types Types 10 Brittle ­ material breaks ­ discontinuous 2. Ductile ­ material flows without breaks ­ continuous Dependant on 1. Temperature 2. Pressure 3. Deformation Rate 4. Composition 1. 04/03/12 Brittle vs. ductile deformation Brittle vs. ductile deformation • Rocks do not all deform similarly Rocks 1. Ductile deformation: continuous deformation 1. Ductile continuous 2. Brittle deformation: discontinuous deformation 2. Brittle discontinuous Ductile: rock does not rock break break Brittle: rock Brittle: breaks or fractures fractures Brittle vs. ductile deformation • Rocks do not all deform similarly Rocks 1. Ductile deformation: continuous deformation 1. Ductile continuous 2. Brittle deformation: discontinuous deformation 2. Brittle discontinuous Original Original sample sample Brittle Brittle Ductile Brittle vs. ductile deformation • Rocks do not all deform similarly Rocks 1. Ductile deformation: continuous deformation 1. Ductile continuous 2. Brittle deformation: discontinuous deformation 2. Brittle discontinuous Brittle deformation is favored by low T and low to Brittle deformation is favored by low T and low to moderate P; moderate P; Ductile deformation is favored by high T and P Ductile deformation is favored by high T and P Ductile and in salt and shale. Ductile and in salt and shale. Brittle/ ductile Ductile Fractures, part 1 (brittle) 14 Faults • Crack WITH relative movement • Recognized by – – – – – Offset, displacement, slip Fault scarp Drag folds Fault breccia, gouge Slickenslides, Slip lineations • Mylonite – actually ductile but depends on T, P and composition Faulting is BRITTLE, and discontinuous in nature! 04/03/12 Offset – really same as displacement PHOTO © STEPHEN MARSHAK W. W. NORTON Faults Brittle 16 features Caused by rapid deformation rate Folding Folding Ductile, Continuous Deformation Produces “Structures” : Like Synclines, Monoclines, Anticlines, Domes And Basins 04/03/12 Folding – continuous deformation, ductile Fold parts and types Anticline - arch Fig. 11.20a STEPHEN MARSHAK Fig. 11.20b STEPHEN MARSHAK Syncline - trough Monocline Fig. 11.20b STEPHEN MARSHAK Map view Fig. 11.21ab When you look at these, Remember Superposition W. W. NORTON Plunging anticline Often, folds aren’t parallel with The Earth’s surface, and they Actually intersect it at an angle… They “Plunge” Plunging fold Fig. 11.20c STEPHEN MARSHAK 23 04/03/12 24 With no weathering, can you see how the layers would connect? 04/03/12 25 04/03/12 26 04/03/12 dome basin 50 k m Dome 27 04/03/12 If a structure is intact, then it’s easy! Usually, they are eroded + incomplete. dome basin If the strata become OLDER towards the center of the structure, then If OLDER It is a DOME! It DOME If the strata become YOUNGER towards the center of the structure, If YOUNGER It is a BASIN! BASIN 28 04/03/12 Fig. 11.20d STEPHEN MARSHAK 11_24c.jpg 30 04/03/12 Fig. 11.08d STEPHEN MARSHAK Ductile flow folding of marble yielding beautiful folds (note the not too beautiful geologist for scale) Folding causes Fig. 11.23 W. W. NORTON Compression or shortening Shearing Fault related Fault bend Drape folding Folding Ductile deformation Slow deformation rate 33 04/03/12 Mountain Building Mt Everest 29,035 ft (8850 m) above sea level 34 04/03/12 Active Mountain Ranges Alpine-Himalayan CordilleranAndes 36 04/03/12 Linear ranges of mountains Orogeny - mountain building event, Orogeny processes that collectively produce a mountain belt Most mountain building occurs at convergent boundaries convergent -Form along current plate boundaries. Form -Ranges within plates generally mark old Ranges continental collisions. continental - As soon as mountain begins “growing” erosion attacks erosion -Can only grow if uplift > erosion 37 04/03/12 Isostasy Concept of a floating crust in gravitational balance •Pushing up = pulling down 38 When weight removed, crustal uplifting occurs •Process is called isostatic adjustment •Wants to be in Isostatic Equilibrium Continental crust < mantle Therefore continental crust floats 04/03/12 Death of a mountain range Erosion and resulting isostatic adjustment of the crust 39 “Initial conditions” Erosion > uplift “Residual Mountains” 04/03/12 Various methods to make mountains 40 04/03/12 41 1. Orogenesis of a volcanic island arc “mountain” – Aleutian type Found in shrinking ocean basins, such as the Pacific e.g. Mariana, Tonga, Aleutian, and Japan arcs Similar process of hotspot volcanoes, Only sustained across Subduction zone 04/03/12 2. Orogenesis along an continental volcanic arc “mountain”Andean-type Initial conditions Thrust-faulting Thrust-faulting Igneous intrusions: Plutons Igneous intrusions: Plutons Underplating Underplating Regional metamorphism Regional metamorphism 42 04/03/12 3. Continent-Continent collisions 43 04/03/12 The collision of India and Asia produced the Himalayas 44 (before) Suture – zone where two land masses are joined 04/03/12 Indian continent impacting into Eurasian continent 45 04/03/12 4. Continental Accretion Small crustal fragments collide + accrete to continental 46 margins Distribution of modern day oceanic plateaus and other submerged crustal fragments 04/03/12 47 04/03/12 48 Accreted crustal blocks are called terranes 04/03/12 5. Mountain building associated with Rifts 49 Continental thinning Normal faults + block rotation Footwall can go up Narrow elongate ranges 04/03/12 ...
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